Electrostatic transducers



July 24, 1956 H. BoUcKE ELECTROSTATIC `TRNSDUCERS 3 Sheets-Sheet. l

Filed March 21 1952 Fill...

INVENTOR /f/NZ .Banen/E.

ATTORNEY `uly 24, 1956 H. BoucKE 2,755,796

ELECTROSTATIC TRANSDUCERS Filed March 21, 1952 z sheets-snaai 2 INVENTO R /f//VZ gamen/E.

ATTORNEY July 24, 1956 H. BoUcKE 2,755,796

ELECTROSTATIC TRANSDUCERS Filed March 2l 1952 5 Sheets-Sheet 3 ATTORNEY United States 2,755,796 ELEcTRosrATIc TRANSDUCERS- The present invention relates to electrostatic transd'ucersV or microphones for converting variations in pres'- sure or mechanical displacement into corresponding changes of electric current or voltage. The invention, among many other uses, is especially applicable to the recording of the pulse and other physiologic pressure variations of the human body for medical purposes.

Capacitative' transducers at present known usually consist of a stationary electrode cooperating with a spaced resilient electrode or diaphragm movable towards and away from said stationary electrode in accordance with the' pressure variations or displacements to be recorded. The capacitance of such transducers is comparatively srall, thus involving the dihculties, when used in connection' with ordinary translating circuits, that the lower limitl response frequency will be insuliicient for recording extremely low frequencies, suchas contained in the pulse beat and other physiologic pressure waves of the human body. Furthermore, when used as a' sound generatoror emitter', such a transducer is able to produce only a limited output power due to its comparatively small elec-y trical'capacitance.

While it is' possible to increase the eiciencyof capacitative transducers by increasing the elective' surface area of the electrodes to provide an increased electric capacitance, this will result in bulky and unwieldy devices of a siz'e prohibitive for most practical applications.

Accordingly, among the objects of the invention is` the provision cf` an electrostatic microphone or transducer which is simple in design and easy to manufacture; which isof small size and weight, While capable of producing considerable capacitance changes as a result of cornpressional, ilexural or torsional forces exert thereon, to alord' the pickup or recording of extremely low frequencies; which has a high elasticity or quick-actingY r e siliency to substantially prevent creeping or hysteresis effects and consequent distortion; which can be used with comparatively low direct current operating voltages com-v pared with electrostatic microphones of known constructions; which is especially suitable as a soundV generator or emitter for use in thesuperaudible or ultrasonic range; which can be designed as a sound receiver or emitter with directive effect in a simple manner; the capacitance changes of which can be converted directly into proportionate and relatively large voltage changes by means of a simple direct current converting circuit, especially suitable for the recording of body sounds and blood or other physiologic pressure variations; and which can generally be used as an efficient electrostatic pickup orr pressure manometer for all purposes where slight changes in pres# sure or mechanical displacement are to be converted into proportionate relatively large changes of electric current or voltage, for indicating, recording and control purposes.

Applicant has discovered that the efficiencyof an elecj trostatic transducer, in particularwithinthe lower frequency range from zero to about l1000 cycles, cabe`co'ri- ISiderably increased by constructing the transdcer'in the atent rice form of av fiat multiple-Wound iiexible condenser section having a` suicient elasticity to afford a proportionate compression andv expansion in accordance with a controlling pressure wave or other varying mechanical force or dis? placement to be recorded. As an example, the electrodes: ot a Wound transducer may consist of a thin springy metal sheet of brass, copper or the like. The dielectric elements or layers interposed between the electrodes are advan tageously inthe form of oiled or other paper, polystyrene sheet or anl equivalent ilexible plastic insulating material. if the condenser section or transducer is operated at temperatures below about 40 C., regular capacitor paper may be used as dielectric material. The elasticity of the section may be improved by special means or methods, such as corrugating or folding the dielectric and/or con-v ducting elements in simple or multiple fashion or by the hermetic inclusion of air strata or layers, in a manner more clearly described in the following.

The invention will be better understood by ref te/rtentze` to the 4following detailed description of a few practical embodiments', considered in conjunction withthe ac'comp'anyingv drawings, forming part of this specification, and wherein:

Figure 1 illustrates diagrammatically a flattened ieX-ible wound condenser' section serving as a transducer element, according to the invention;

Figure 2 shows an alternative construction inthe form of amultiple stack folded transducer of this type;

Figure 3 shows an arrangement for pulse waveirc'cordingv by the aid of a transducer according to thel invention;

Figure 4 illustrates an alternative arrangement foi` pulse wave recording by the use of a transducer accordiugto the invention;

Figures Sand 6 are plan and side views, respectively, of an improved device for pulse or other physiologic pressure recording embodying a pair of transducer elements according to the invention;

Figure 7 shows another device according to the'inven# tion especially suitable for heart or other body sound recording;

Figures 8 and 9 are electrical circuit diagrams suitable tory converting the voltage variations produced by the transducer' of the inventiony into proportionate current changes;

Figure 10 shows an embodiment of the capacitative transducer of the invention in the form of a phonographic pickup or recording device;

Figure ll is a circuit diagram suitable for use in connection with the pickup shown in Figure 10;

Figures l2 and 13 are plan and side views, resp ectively, of an electrostatic telephone or sound reproducer con# structed in accordance with the invention;

Figure 14 is a circuit diagram of an energizing circuit for use with the telephone or reproducer shown in Figures l2 and 13;

Figure 15 shows another modicationof asoundre-1 producer or generator constructed in accordance with the principles of the invention;

Figure 16 is an energizing circuit for use in connection with the device shown in Figure l5;

Figures l7 to 19 illustrate schematically improved features of construction for a transducer or microphone of the type according to the invention;

Fig. 19a is a' partial perspective view of a transducer unit, with parts shown Vbroken away, illustrating further features of improvement according to the invention;

Figure 20 is a view of another form of an electrostatic pickup or sound reproducer embodying the Aprinciples of the invention;

VFigure 2l is a sectional view'takenl online 21;'21'1ofC Figure v20;

W W Y i. u

Figure 22 illustrates a further modification of a pulse wave recorder constructed according to the invention;

Figure 23 illustrates a control or calibrating device for4 use in combination with a pulse wave recorder according to the invention; and

Figure 24 is a circuit diagram showing an improved control circuit for varying the output voltage of a capacitative transducer according to the invention and similar types.

Like reference characters identify like parts throughout the several views of the drawings.

' Referring more particularly to Figure l, there is shown diagrammatically a capacitative transducer according to the invention comprising a flat wound and relatively thin capacitor section fabricated by either interleaving a pair of flexible conducting strips 11 and 12 and dielectric strips 13 and 14, such as of paper or a suitable plastic, or by winding a composite or laminated strip comprising the elements 11, 12, 13 and 14 into a attened roll or section, as shown in the drawing.

fFigure 2 shows an alternative construction, wherein a composite strip is folded back and forth to provide a multiple stack or condenser section.

Figure 3 shows an embodiment of a capacitance transducer of the type according to the invention, for pulse wave recording, comprising a iat wound transducer or condenser section 10 mounted in a thumb sleeve 15 of leather, rubber or the like, the transducer being separated from the interior of the sleeve by an insulating layer or spacer 16 also consisting of leather, rubber or an equivalent insulating material. One of the electrodes of the transducer 10 is connected, through suitable terminal leads applied in a known manner customary in the manufacture of wound condensers, with the outer electrode of a concentric cable 17, the cooperating electrode of the transducer being connected to the central cable conductor 18, for connection to a suitable translating circuit or amplifier. Since the sleeve is comparatively rigid, the volume variations of the thumb caused by the pulse beat are transformed into proportionate compressions ofthe wound elastic transducer. Recordings made of the capacity changes of a device of the type shown have yielded pulse wave forms with all details and a degree of fidelity hardly obtainable with other recording means.

A further advantage of a device of the type described is the fact that movements of the thumb will be substantially without eiect on the recording of the pulse wave, provided the connecting cable has a sufcient exibility. In order to convert the mechanical movements or volume changes into proportionate electric current or voltage variations, any of the direct current conversion circuits known in connection with condenser microphones or pickup devices may be employed by providing an adequate bias voltage applied to the transducer through a high ohmic operating resistance. On account of the relatively high capacity of the transducer, i. e. practically from about 20,000 to 50,000 mmfd., use of a resistor of from 50 to 100 megohms will result in a lower limit recording frequency of from 0.2 to 0.5 cycle, suicient for the recording of the fundamental component of the pulse beat waves of the human body. Where only very small capacitance changes are available as in standard electrostatic microphones or transducers, direct current operation is unsuitable for many reasons and conversion must be eifected by way of a high frequency oscillator and translating circuit. As a matter of fact, due to the extreme sensitivity or high capacitance changes of a transducer as described, a direct connection to the control grid and cathode of a single amplifier tube may result in an output sufficient to operate an indicating or translating device, the transducer or capacitance in this case modulating the voltage across the grid leak resistance of the amplifier.

The sleeve 15 in Figure 3 may be held in place by means of a strap or bandage 20 and secured by the aid of a cross-pin 21 inserted between adjacent windings in a manner readily understood.

Referring to Figure 4 there is shown a modication of a transducer according to the invention for pulse wave recording being especially suited for application to the wrist or upper arm by the aid of a strap or bandage ZZ. The bandage may be secured or held in place in any suitable manner, such as by the narrow end portion 23 cooperating with a clasp or buckle 24, in a manner well understood.

Referring to Figures 5 and 6, the transducer or pick-up device is shown in the form of a probe which may be interposed between a strap or bandage and the part of the body where a pulse beat, blood pressure or the like is to be recorded. For this purpose, the transducer 10 is mounted upon a support of plate 25 of comparatively rigid insulating material. There is furthermore shown an auxiliary transducer 26 similar to and mounted on top of the transducer 10, to serve for the special purpose of applying an articial control or calibrating pulse to the device. For this purpose, the transducer 26 is charged through its connecting cable 27 by an electric voltage pulse of a given magnitude to result in a predetermined compression of the main transducer 10.

There is furthermore shown in Figure 6 an elastic pressure member 28 axed to the rear surface of the support 25 and consisting, for instance, of sponge rubber or the like, to allow the application of the device to the body under a denite steady or continuous pressure. This steady pressure may be simply determined by measuring the capacity of the transducer. The latter varies within several hundred percent as a function of the steady pressure, ranging, according to a practical example, from 8000 mmfd. in the unloaded condition to 25,000 mmfd. in the loaded condition at a pressure of mm. of mercury. For this measurement, all that is required is a suitable milliamrneter for 60 cycles, whereby, by the use of an available network voltage, the current indicated will be a direct measure of the capacitance. As is understood, the transducer in this case should be sui- `ciently insulated from the body. v In order to enable the measurement of the steady pressure simultaneously with a blood pressure measurement or the recording of the pulse wave, it is furthermore possible to use an additional transducer arranged adjacent to the pulse recording transducer. It is furthermore possible to apply the transducer for recording the pulse wave closer to the periphery of the body, such as at the wrist, while measuring the blood pressure at the upper arm by the aid of a separate transducer.

Referring to Figure 7, there is shown an arrangement being especially suitable for recording heart beats or other body sounds or murmurs, such as those near the larynx, and comprising a multiplicity of transducers 30a, 30h, 30C, 30d and 30e, in the example shown, arranged in juxtaposed position upon a base in the form of a 'strap or bandage 31. The arrangement may be secured to the body by means of an elastic garter 32 or in any other suitable manner. If such device is applied to the chest, comparatively large capacity variations are produced during the relatively slow inspiration and expiration cycles. Depending upon the frequency characteristics of the amplifier or coupling elements, the breathing frequency or the heartbeat frequency may be recorded in this manner.. Such a discrimination is already obtained with a given terminal impedance of the transducer. The several transducer units 30a, 301), 30C, 30d, 30e may be electrically connected either in series or parallel or in a series-parallel arrangement.

It is furthermore possible to secure the device near the heart by means of adhesive tape, in such a manner that it will follow the breathing movements in its entirety, while only the heart sounds will aifect or inuencethe transducer.

Referring to Figures- 8"and 9,Y there4 are: 'shown-'suitable translating circuitsv for connecting` ay transducer ofthe typ'e described to the input of a Vacuum tub'efamplifie. In-.Figure 8, thetransducer indicated diagrammatically at lis charged through a high ohmic operating resistance 36 by a battery or equivalent direct current voltage source 37 having, as an example, a voltage oflOO-volts. As a result of the varying compression, the resultant capacitance changes 'of the transducer are Yconverted into corresponding voltage variations whichare` appliedito the control grid of the amplifier tube 38` through the coupling or blocking condenser 40 in series fwith -a'further resistance 41. Resistance 42 constitutes the normal grid-leak resistance for the tube.v

Resistance 41 the value of which isfa multiple of the resistance 42 serves for the purpose of increasing the time constant of the charge and `discharge of the transducer, in such a manner as to afford a recording of the fundamental component of the pulse-orheartbeat waves of the human `bod-y. A further condenser 43 maybe connected across the resistance 41 in order to record the higher frequency components of the pulse beat wave within the audible range, such as murmurs and noises -occurring during a -blood pressure measurement, the condenser 43 forming a shunt path for these frequencies. The use 'of the condenser 43 is furthermore advisable-in order to compensate the weakening effect upon the high frequency components by the inherent grid-cathode capacity of the tube.

In the modification according to Figure 9, the series resistance 36 is replaced by a condenser 45, in which case the blocking Vcondenser 40 may be dispensed with. The capacitance of this condenser 45 is preferably of the same order as the capacity of the transducer, that is, from 10,000 to 50,000 mmfd.

Referring to Figure 10, there is shown schematically a construction of an electrostatic transducer according to the invention for phonographic sound pick-up or-reproduction. For this purpose, a stylus 46 cooperating in a known manner with a phonograph Vrecord is mounted to rotate or oscillate in the direction of the arrows about a pivot 47. As a result, the rigidv arm 48 at the opposite lside of the pivot will press alternately against one and the other of a pair of capacitative transducers 10a and 10b constructed in accordance With the invention and mounted on both sides of the arml 48in an inverted U-shaped casing or support 49. By. such a push-pull arrangement, any nonlinearity in the electric voltages resulting from the pressure changes, especially in the case of high amplitude swings, will be substantially minimized or cancelled.

Figure 1l shows an electric translating circuit forn a push-pull pickup of the type according to Figure 10.l The transducer 10d is charged by a first battery 37a through a condenser 45a. The resulting voltage variations are applied through the high ohmic resistance 41a to the grid ofthe amplifier tube 38 in a manner similar -to Figure 9. In a. likemanner, the transducer 10b is charged by way of thecondenser 45h by the battery 37b and they resulting electric voltage changes are applied to the grid of the tube 3S by way of resistance 42a.

If both transducers 10m and 10b were compressed simultaneously, the voltage variations occurring in an opposite phase due to the opposite polarity connection of the batteries 37a and 37b, would cancel each other in their effect upon the grid of the amplifier. In practice, however, due to the push-pull operation of the transn ducers by the arm 48, both alternating. voltages are added in their effect upon the amplifier tube 38. `The condensers 43a and 43k serve for the purpose of increasing or boosting the higher frequency components which with this type of pickup are more or less weakened compared with the lower frequency components, wherebytoobtain a more uniform and-faithful sound reproduction. The efiiciency of this arrangement, the dimensions .fora practicalexample ofwhich are given in the-'following',zi'sf'so high that, when using a sensitive amplifier, a single tube or stage may be sufiicient to produce sufficient output power for operating a loudspeaker. In a practical ex; ample, the D. C. or bias voltage had al value from to 300 volts, the transducers 10a and 10b and the series condensers 45a and 45b had a capacity from 5,000 to 15,000 mmfd., the resistances 41a and-41b had a value from'- 50 to l100 megohms, the value of the grid-leak resistance 42 was from .7 to 1.5 megohms, and the capacitance of the condensers 43a and 43b had a value of from 30l to mmfd.

Figures 12 and 13 show another embodiment of an electrostatic transducer 10 according to the-invention in the form ofl an earphone or telephone receiver. The transducer shown has a square shape, the sides of the square being about 15 mm. and the thickness being about 1.5 mm. The transducer is sandwiched orinterposed between two thin rubber discs 50 and 51 which also include an insulated connecting cable 17. The exciting voltage may be applied by means of a circuit shown in Figure 14 which may be the output or power-stage of an audio amplifier. The transducer 35 is connected through-the high ohmic operating resistance 52 to the anode andthe cathode of the amplifier tube 53, to provide a -constant or steady biasvoltage by the plate voltage source of the tube. In order to avoid a weakening A,of the higher frequency components, the resistance 52 may be shunterd by a condenser S4 having a capacitance equal to a fraction of the capacitance of the transducer 35. Furthermore, the choke coil 55 in the direct current supply lead to the anode may have a sufficiently high self-induction to cause resonance for the upper limit frequencies with the capacitative load of the anode circuit.

Referring Vto Figure 15, there is shown an' alternative arrangement of a sound generator or emitter' comprising two transducers 10a and 10b operating in push-pull and actuating the diaphragm 56 of a loudspeaker or equivalent device. In the example shown, the diaphragm' 56is mounted in a conical housing 57, in a manner customary with loudspeaker designs, a perforated'plate 58 being secured yto the opening of the housing 57 and `'carrying 'a central supporting member 60 for the transducer 10i?, the transducer 10b being directly mounted upon the housing 57,- as shown. n o

Figure 16 shows a circuit for energizing a push-pull sound generator of this'type by the output stage 53 of `-a sound amplifier. For'this purpose, the secondary winding 'of the output transformer 61 issubdivided into two equal sections 62 and 63 each of which vserves to excite one ofthe transducers 10a and 10b, respectively. The common junction point of the transducers 10a and 10b is connected to the center point of the secondary winding of the transformer 61 by way of the plate voltage source of the amplifier, as shown. As a result, both transducers will be biased by the anode vsupplyvoltage in the'same direction, whilethe alternating exciting voltages will be in phase opposition. Suitable resistances may be inserted between the windingsections 62 and 63 and the respective transducersSSa and 35b,l respectively, to modify the frequency response characteristic.

As will be understood, a plurality of transducersvof dierent size and characteristics may be combined-to'ob`= tain a desired over-all frequency response curve, irifa manner well known.

if a transducer according, to the inventionis sofconstructed and arranged in a small space 'With manyrelatively'tightly wound turns or folds subdivided into Agrtuips or sections to result in a high elasticity, it may beused to receive or transmit oscillations within the supersonic range. Such a multiple-section transducer may foi` instance be obtained by a construction shown'in Figure 17, wherein aicmposite'condenser strip 10 is wound or folded concertina-fashion infthe'forr'n of adjacent multiplel a desired resonance effect.

the'transducer are preferably connected by a curved elastic ribbon or sheet 66 having its ends secured to thc support 65, in the manner shown in the drawing.

According to a further feature of the invention, the

dielectric and/or conducting elements of the transducer `may be constructed to have an increased inherent elasticity, such as by corrugating as shown in Figure 19. For

Vgood results or highk elasticity, the corrugations should be ofthe order and not greater than about two to five` times the paper or foil thickness. An especially high elasticity and at the same time a considerable rigidity of the complete unit, especially suitable for use in the supersonic field, may also be obtained by the use of rubber lfoil either in plain or corrugated form,

In order to increase the sensitivity or efficiency for 'a given operating frequency, in particular for applications in the supersonic field, it is advantageous to provide an oscillatory circuit by connecting the transducer in parallel or series with a suitable induction coil to obtain Such an effect may also be obtained to a certain extent mechanically by providing a rigid cover plate S or 66 having suitably spaced openings or apertures, to cause thetransducer to oscillate within a predetermined partial frequency range or ranges. In a loosely wound condenser section of the type used as a transducer according to the present invention, the included air is responsible to a considerable degree for the elasticity of the section. If such a section is subjected to a certain pressure over a prolonged period the air will be expelled gradually and will be sucked in `again upon release of the pressure, due to the elasticity of the `condenser elements or winding turns. In order to prevent an adherence or sticking of the elements t0 one another, it is proposed to coat the elements, that is, the insulating and/or conducting strips or foils with a thin layer of a finely divided insulating material, such as talcum powder or the like. This coating is preferably applied in such a minute quantity as to result in an extremely thin layer sticking to the surface of the elements by the action of adhesive forces. Experiments with a condenser section made in this manner have shown that, by the use of extremely small amounts of powder, effective layers or strata of air may be included in the section to insure a sufficient elasticity under considerable -pressure amplitudes or variations.

Furthermore, in order to utilize the enclosed air as the primary means to provide the required elasticity of the section and to substantially avoid hysteresis effects, the Wound transducer section is preferably hermetically sealed at its edges, such as by dipping the ends of the section in a liquid sealing material which becomes solid upon cooling or, when using metallized paper or an equivalent flexible insulating material, by applying a layer of solder to or spraying the edges of the section by the aid of a metal spray gun, whereby at the same time to provide a means for attaching terminal leads or wires by soldering, clamping, or in any other suitable manner. Fig. 19a is a partial perspective View, with parts shown broken away, of a convolute capacitative transducer according to the invention illustrating, by way of example, the sealing of the unit and coating of the layers with a film of finely-divided powder as described hereinabove. For this purpose the ends of the flat wound unit lidare sealed by a suitable compound, as shown at a, and applied by a dipping process or in any other suitable manner.` Furthermore, the metal layers or foils 11 and 12 separated by the insulating layers 13 and 14 are shown coated with a film of insulating powder 12a and 13a, respectively, the spacing between the layers being exaggerated for the sake of illustration. As a result of the relatively loose winding of the convolute condenser, the elements, that is in particular the metal layers 11 and 12, acquire an added flexibility about the outer or bent portions acting as pivots, thus resulting in an increased elasticity and providing a quick-acting restoring force adapted to follow the impressed pressure variations to be coverted into proportionate capacitance changes of the condenser.

As a result of the hermetic sealing of the flat wound condenser section, a plurality of layers or strata of air will be included between adjacent winding turns to provide an increased cushioning effect or quick-acting resiliency, enabling the completed unit to follow any applied compressional, torsional or flexural` stress, substantially without any creeping or hysteresis effects, to thereby insure a faithful conversion of the pressure variations into proportionate voltage changes. In other words, in the condenser or transducer according to the invention, contrary to existing practice in the manufacture of electrical condensers, the air is purposely retained in the unit without any impregnation, to provide a final flexible element having adesired quick-acting resiliency or elasticity capable of following the relatively rapid changes of the pressure or other input magnitude to be converted.

Referring to Figures 2G and 2l, there is shown an alternative construction of an electrostatic transducer according to the invention to serve as a pickup for use in sound reproducing devices or phonographs. In the embodiment shown, the flexible condenser section or transducer is subjected to a torsional rather than compressional force in accordance with the magnitude to be converted, that is, the oscillations of a needle or stylus of a phonographic reproducer. There is shown for this purpose an inverted U-shaped support of a light-Weight insulating material, such as a suitableplastic, said support being provided in both of its legs with a pair of slots 71 and 72 inclined at an angle to the symmetry plane of the support 70, as shown in Figure 2l. A fiat flexible wound condenser section or transducer 10 is mounted at its opposite ends in the slots 71, 72, such as by a press fit engagement, gluing, or in any other suitable manner. The central portion of the section 10 in turn engages a fork-like carrier 73 for a needle or recording stylus 74. The carrier 73 has an upper extending end 75 engaging or camming with an elastic member 76 consisting of rubber or equivalent material and fixedly mounted cantilever-fashion in the yoke portion of the support 70. Member 76 is so adjusted as to normally apply a steady torsional force upon the section 10 with respect to its central position indicated by the dot-dash line in the drawing. In this manner the torsional vibrations of the section 10 caused by the oscillations of the stylus 74 are limited to a single direction from the center or zero line. Without any steady torsional bias of the section, the device would act as a frequency doubler in converting the oscillations of the stylus 74 into corresponding voltage changes in the associate electrical circuit.

Referring to Figure 22, there is shown another modification of a pulse wave recorder, wherein the transducer 10 r is mounted in a depression of a protective casing 78 and secured by gluing or in any other suitable manner. The side of the transducer 10 opposite to the bandage or strap 8f) and facing the body is covered by a thin rubber skin 81 or equivalent material, such as a fabric impregnated with rubber, and secured to the casing 78. A thin rigid disc 82 is interposed between the section 10 and the skin S1, to protect the transducer against damage. Item 83 is a conical probe or sensing element especially suited for application of the recorder to an artery for pulse wave recording.

Referring to Figure 23, thereis shown a combination of a pulse microphone or recorder with a control or calibratingdevice. The latter consists of two. relatively flat casings 84and 85 having flexibleand elastic walls of any suitable material and being hermeticallyconnected through a exible tube 85. By applying. apressure pulse, by digital pressure or a suitable pulse generator, to the casing 85 until the oppositewalls engage eachother, the casing 84 will be expanded and affectfthe transducer in the same manner as a pulse beat. Given a Vdefinite steady pressure, there is thus produced a well-defined effect upon the transducer, whereby the recordercan `be checked and adjusted at anytime. This has the further advantage that the pulse beat waves of different persons may be compared with a normal or CalibratingV pulse.

In the use of condenser microphones or equivalent transducers, such as those of the type described, a high time constant and corresponding .low limit recording frequency is obtained by connecting .a large auxiliary capacity in parallel to the microphone vor transducer and byv employing a high operating or series resistance. In arrangements of this type, it is often desirable to` effect an amplitude control directly at the transducer or input of the lirst stage of the amplifier, rather than in the anode circuit of the first or a subsequent amplifier stage. The volume control resistor used for this purpose must necessarily have avery high value and as a result entails the disadvantage of weakening the higher frequency components due to the effect of the inherent capacity of the connecting leads between the control resistor and the input of the amplifier. Itis not possible to compensate this frequency distortion by an inverse variation of the amplier frequency characteristic for the reason that the weakening of the higher frequency varies considerably with the setting or value of the control resistor. Thus, in the low volume or Zero position of the resistor the amount of the resistance is so small that the capacitative load of the connecting leads will be negligible. In the opposite or maximum position, on the other hand, the lead capacity is connected in parallel to the considerably higher auxiliary capacity in parallel to the transducer so as in turn to be negligible as far as the frequency characterisic is concerned. In the intermediate positions of the control resistor, however, the eect of the lead capacity on the frequency characteristic will be more or less pronounced.

According to the invention, a compensation of the frequency distortion caused by the capacitative loadon the control resistor, is effected by connecting the auxiliary capacity in parallel to the transducer `through an auxiliary variable resistor, the latter being operatively coupled with the control resistor, in such a manner that, in the positions zero and maximum of the control resistor, the auxiliary resistor has a value equal to. zero and will assume a maximum in the central position of the control resistor. Furthermore, the auxiliary resistor is chosen to have a value of the order of the alternating current impedance of the auxiliary capacity for the frequency at which the weakening of the high frequency components begins to become appreciable in the central position of the control resisto-r. As a result, the weakening of the higher frequency components by the lead capacitance will be substantially compensated by the -effect of the auxiliary resistor on the parallel capacity of the transducer.

Referring to Figure 24, there is shown a control `circuit for an electrostatic microphone or transducer of this type. The microphone 35 may be a wound condenser section in the form of a body sound microphone or transducer, as described. The microphone 3S is connected in known manner to a direct current voltage source 37 in series with van operating-resistance 36. The alternating current voltage produced by the pulse variations or `other input magnitude is applied by way of Vthe .blocking condenser 99 to the volume control resistor 91 havingY a variable contact '92 to provide an output voltage of vdesired magnitude applied to the grid and cathode of the lamplifier tube 38 forming the input stage ofvv anaudio frequencyfampliiier. 93 indicates the inherent capacity of `the-connecting leads between the controlresistorf 91 and the tubew38 responsible for the weakeningof'the higher frequency'component-s, as pointed out. The auxiliarycapacityconnected in parallel to the microphone '35 together with --theyresistance 36 serves to provide a sufficiently hightimeconstant or lower limit recording frequency, inthe manner'pointed out.

In @accordance with the present improvement, the auxiliary/capacityvis divided into two parts, i. e. axedjcapacity 94 and -afunther capacity 9S-in series with la variablev4 auxiliary resistorV 96, both connected'in parallel to the microphonewor-transducer 35. The volume control resistor 91 and the auxiliary resistor 96 are mechanically ,coupied,- as indicated `at 97, in suchfa manner that resistor 96 will bezero'in the extremefpositions of the vresistor 91andwiil be a maximum in the central position of the resistorl, asshown inthe drawing. This object is lobtainedinva-simplemanner by connecting both ends of theresistorr96` toone'terminal of the condenser micro- .phone.3 5Y and by. connecting the variable contact 98' to the other terminalffo-f the microphone through the capacity 95, as shown in the drawing.

In an arrangement of this type, the decrease ofthe higher frequency components at or near the central positions of the control resistor 91 is substantially compensated by the circuit 95, 96 by a relative accentuation of thefhigher'frequencies, in such a manner as 'to substantially compensateV for frequency distortion causedby the leadcapacitance 93.

The circuit 4described has manyadvantages in practice; Thus, if the microphone is connected to anV electrocardiographic` amplifier having anamplitude control-in the plate 'circuit of the first or a subsequent amplifying stage, excess control or distortion ofthe amplifier is prevented even for relativelylarg'e amplitude savings -of the microphone, in particular when using a transducer constructedy in accordance with thepresent invention. Furthermore, this circuit makes it possible'to compare two or'more microphones orftransducers, in which case the amplitude control or gain ofthe ampliiier must remain constant or unchanged.

There is thus' provided by the'invention a simple and highly eiicie'ntV electrostatic transducer or pickup devicefconsisting of a plurality of interleaved flexibleconducting ,and insulating: elements wound or folded into a exible. andunimpregnated condenser section having a sufficient elasticity or quick-acting resiliency to be able tovrespond tothe instantaneous pressure variations `in accordance with an input magnitudeto be recorded, sub1 stantially Without any hysteresis effect.

The condenser-section itself may -be-either of the inductive typeV wherein the insulating strips completely covered the metal strips Vand the terminal leads are con nected-in any-suitable'manner such as by means of conducting tabs inserted between adjacent windings, or other-` wise electrically connected with the respective electrode strips. Alternatively, the ledges ofthe conducting strips may be` arranged to alternately project to or beyond the opposite edges of the conducting strips in the manner of a vnon-inductively wound condenser. This construction has lthe advantage that the condenser may be completely sealed Vby soldering orotherwise connecting the projecting edges-of theelectrodes, to include layers of air between the adjacent turns and to improve the resiliencyor elasticity of the completed unit in the manner pointed out hereinabove.

In a preferredconstruction, the condenser or transducer is constructed -with a pair of metallized insulating strips yof. paper, polystyrene, or another suitable insulating material, eachk strip having-an insulated margin free from thev metal coating andthe strips beingarrangedwith their margins at opposite ends of -theunitorsection. After winding the section, it can be sealed in a simple 1 1 manner by spraying the edges with nely divided metal to adhere to and seal the edges of the respective metal coatings. At the same time this will afford a simple means of applying suitable terminal leads to the unit.

Such a unit is characterized by its elastic properties and other uniform operating characteristics, whereby to enable its manufacture to closely exacting specifications or tolerances. In other words, contrary to the demands in normal condenser construction, that is the provision of a rigid unit including a solid or liquid and non-compressible impregnating medium filling the interstices between adjacent winding turns, the present invention utilizes a unit Wound in a relatively loose or flexible manner and including a compressible medium, such as air, between the elements or winding turns of the unit to enhance its elastic and other desirable properties. Problems of electric discharge or losses caused by the air within the unit are practically non-existent in the case of a transducer or pickup device according to the invention, due to the relatively low operating voltage and power transfer involved and the absence of any excess surge or other disturbing voltages liable to damage and incapacitate the condenser as in the case of high voltage or power applications.

As will be understood, the output current varying in accordance with the capacity changes of the transducer may be utilized for energizing an indicator, recorder or control device of any suitable type known in the art in connection with mechanico-electric translation systems. Thus, in recording pulse waves, heartbeats or the like, recording may be effected in accordance with any of the methods known, such as by means of an optical galvanometer cooperating with a photo-sensitive paper, by an electro-magnetically driven pen producing an inked record on ordinary paper, by means of a direct-writing electrocardiographic recorder or galvanometer with an electrically heated stylus etching the meter movement by the heat of the stylus in the surface of a special plastic coated paper, by means of a cathode ray oscillograph, or any other suitable type of recording device known.

In the foregoing the invention has been described with reference to a specific illustrative device. It will be evident, however, that numerous variations and modifications, as well as the substitution of equivalent elements for those described and shown herein for illustration, may be made without departing from the broader scope and spirit of the invention as defined by the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a limiting sense.

I claim:

l. 'An electrostatic transducer constituted by a relatively at electrical condenser comprised of interleaved flexible conducting and insulating elements being relatively`loosely Wound into a attened convolute unit having sufficient elasticity to allow of compression and eX- pansion in proportion to variations of a mechanical force applied thereto for conversion into proportionate capacitance changes of said condenser.

2. An electro-static transducer constituted by an electrical condenser comprised of interleaved ilexible'conducting and insulating layers being relatively loosely wound into a flat convolute condenser unit having a sufficient elasticity, whereby to allow of compression and expansion of said unit in proportion to variations of a mechanical force impressed thereon for conversion into proportionate capacitance changes of said condenser, and a film of talcum powder applied to at least one of any two adjacent layers of said unit.

3. An electrostatic transducer constituted by an electrical condenser comprised of interleaved flexible conducting and insulating layers being relatively loosely wound into a fiat convolute condenser unit having a suliicient elasticity, whereby to allow of compression and expansion of said unit in proportion to variations of a mechanical force `irnpressed thereon for conversion into proportionate capacitance changes of said condenser, and means to seal the opposite ends of said unit.

4. An electrostatic transducer constituted by an electrical condenser comprised of interleaved flexible conducting and insulating layers being relatively loosely wound into a flat convolute condenser unit having a suicient elasticity, whereby to allow of compression and expansion of said unit in proportion to variations of a mechanical force impressed thereon for conversion into proportionate capacitance changes of said condenser, and a film of finely-divided insulating powder applied to at least one of any two adjacent layers of said unit.

5. In a device for recording physiologic pressure varialions, a capacitative transducer constituted by an electrical condenser comprised of interleaved flexible conducting and insulating layers being relatively loosely wound into a fiat convolute condenser unit having a sufficient elasticity, to allow of compression and expansion thereof in proportion to variations of a mechanical force impressed thereon for conversion into proportionate capacitance changes of said condenser, supporting means for applying said unit to the human body to impress thereon the physiologic pressure variations to be recorded, and means for converting the resulting capacitance variations of said condenser into proportionate electric current changes.

6. ln a device for recording physiologic pressure variations, a capacitative transducer constituted by an electrical condenser comprised o-f interleaved flexible conducting and insulating layers relatively loosely wound into a flat convolute condenser unit having a suiiicient elasticity and a film of finely-divided insulating material applied to at least one of the said layers, whereby to allow of compression and expansion of said unit in proportion to variations of afrnechanical force impressed thereon for conversion into proportionate capacitance changes of said condenser, supporting means for applying said unit to the human body to impress thereon the physiologic pressure variations to be recorded, and means for converting the resultant capacitance variations of said condenser into proportionate electric current changes.

7. in a device for recording physiologic pressure variations, a capacitative transducer constituted by an electrical condenser comprised of interleaved flexible conducting and insulating layers being relatively loosely wound into a flat convolute condenser unit having a sufficient elasticity and means to hcrmetically seal the opposite ends of said unit, whereby to allow of compression and expansion of said unit in proportion to variations of a mechanical force impressed thereon for conversion into proportionate capacitance changes of said condenser, supporting means for applying said unit to the human body to impress thereon the physiologic pressure variations to be recorded, and means for converting the resultant capacitance variations of said condenser into proportionate electric current changes.

8. In a device for recording physiologic pressure variations, a capacitative transducer constituted by an electrical condenser comprised of interleaved flexible conducting and insulating layers being relatively loosely Wound into a fiat convolute condenser unit having a sufficient elasticity, whereby to allow of compression and expansion thereof in proportion to variation of a mechanical force impressed thereon for conversion into proportionate capacitance changes of said condenser, supporting means including auxiliary pressure applying means, for applying said unit to the human body with a predetermined steady pressure and to impress thereon the physiologic pressure variations to be recorded, and means for converting the resulting capacitance variations of said condenser into proportionate electric current changes.

9. In a device for recording physiologic pressure variations, a capacitative transducer constituted by an electric condenser comprised of interleaved flexible conducting and insulating layers being relatively loosely wound into a `llatt'ened convolute condenser unit of sufiicient elasticity,

References Cited# in the le of this patent NITED STATES PATENTS Comstock et al. May 11, 1926 Rieber Oct. 9, 1934 Speaker et al. Nov. 2, 1948 Lindenberg et al Oct. 28, 1952 

